Purification of Carbon Dioxide

ABSTRACT

Refrigeration duty in a carbon dioxide purification unit (CPU) operating at elevated pressure and sub-ambient temperature can be provided in at least a first part by indirect heat exchange against at least latent heat of at least one liquid first refrigerant, preferably carbon dioxide liquid(s) produced in the CPU, thereby typically evaporating the liquid(s), and a second part by indirect heat exchange with sensible heat energy alone of a second refrigerant. The second refrigerant may be nitrogen gas imported from an integrated cryogenic air separation unit (ASU) or carbon dioxide liquid exported from the CPU, cooled and returned to the CPU. One advantage is that total power consumption of the CPU and an integrated ASU is reduced.

BACKGROUND OF THE INVENTION

The present invention relates to a method of purifying crude carbondioxide gas using a low temperature carbon dioxide purification process.Specifically, the present invention resides in the manner in which therefrigeration duty for such a method may be provided in order to reduceoverall energy consumption. The present invention has particularapplication in processes that capture carbon dioxide from flue gasproduced in an oxyfuel combustion process to raise steam for electricpower generation, in which oxygen for the combustion is produced in acryogenic air separation unit (ASU).

It has been asserted that one of the main causes of global warming isthe rise in greenhouse gas contamination in the atmosphere due toanthropological effects. The main greenhouse gas which is being emitted,carbon dioxide (CO2), has risen in concentration in the atmosphere from270 ppm before the industrial revolution to the current figure of about378 ppm. Further rises in CO2 concentration are inevitable until CO2emissions are curbed. The main sources of CO2 emission are fossil fuelfired electric power stations and from petroleum fuelled vehicles.

The use of fossil fuels is necessary in order to continue to produce thequantities of electric power that nations require to sustain theireconomies and lifestyles. There is, therefore, a need to deviseefficient means by which CO2 may be captured from power stations burningfossil fuel so that it can be stored rather than being vented into theatmosphere. Storage may be deep undersea; in a geological formation suchas a saline aquifer; or a depleted oil or natural gas formation.Alternatively, the CO2 could be used for enhanced oil recovery (EOR).

Oxyfuel combustion seeks to mitigate the harmful effects of CO2emissions by producing a net combustion product gas consisting of CO2and water vapour, by combusting fuels such as carbonaceous fuels,hydrocarbonaceous fuels and biomass, in an oxygen-rich (e.g. more than20 wt %) atmosphere. An oxyfuel process for CO2 capture from apulverised coal-fired power boiler is described in a paper entitled“Oxy-combustion processes for CO2 capture from advanced supercritical PFand NGCC power plants” (Dillon et al; presented at GHGT-7, Vancouver,September 2004), the disclosure of which is incorporated herein byreference.

Oxygen for the combustion is typically supplied from an ASU. However,using pure oxygen for the combustion would result is a very highcombustion temperature which would not be practical in a furnace orboiler. Therefore, in order to moderate the combustion temperature, partof the total flue gas generated in the combustion is recycled, usuallyafter cooling, back to the burner. The net flue gas is then processed toproduce carbon dioxide for storage or use.

Oxyfuel combustion produces raw flue gas containing primarily CO2,together with contaminants such as water vapour; “non-condensable”gases, i.e. gases from chemical processes which are not easily condensedby cooling, such as excess combustion oxygen (O2), and/or O2, N2 andargon (Ar) derived from any air leakage into the system; and acid gasessuch as SO3, SO2, hydrogen chloride (HCl), NO and NO2 produced asoxidation products from components in the fuel or by combination of N2and O2 at high temperature. The precise concentrations of the gaseousimpurities present in the flue gas depend on factors such as on the fuelcomposition; the level of N2 in the combustor; the combustiontemperature; and the design of the burner and furnace.

Water and the acid gases are removed from the flue gas which iscompressed to elevated pressure. The gas is then purified to producepure carbon dioxide in a sub-ambient or low temperature carbon dioxidepurification unit (CPU). The carbon dioxide may be purified bydistillation or by partial condensation and phase separation.

In general, the final, purified, CO2 product should ideally be producedas a high pressure fluid stream for delivery into a pipeline fortransportation to storage or to site of use, e.g. in EOR. The CO2 mustbe dry to avoid corrosion of, for example, a carbon steel pipeline. TheCO2 impurity levels must not jeopardise the integrity of the geologicalstorage site, particularly if the CO2 is to be used for EOR, and thetransportation and storage must not infringe international and nationaltreaties and regulations governing the transport and disposal of gasstreams.

FIG. 1 depicts separate prior art CPU and ASU processes that may beintegrated with an oxyfuel combustion unit. In the CPU, a stream 100 ofCO2 rich flue gas from an oxyfuel combustion unit (not shown) iscompressed in a compressor 102 to about 30 bar (3 MPa) to produce astream 104 of compressed flue gas. Stream 104 is dried in drier unit 106and fed as stream 108 to the main heat exchanger 162 where it is cooledto about −32° C. by indirect heat exchange to produce partiallycondensed stream 110. A stream 110 is fed to a first phase separator 112for separation into a vapour stream 114 and a stream 116 of carbondioxide liquid.

The vapour stream 114 from this first separator is fed to the main heatexchanger 162 where it is further cooled to about −54° C. and partiallycondensed. The partially condensed stream 124 is fed to a second phaseseparator 126 where it is a separated into a vapour stream 128 and astream 132 of carbon dioxide liquid. Stream 128 containing thenon-condensible gases is warmed in the main heat exchanger to producestream 130 and may be vented at pressure, heated and expanded for powerrecovery, or expanded within the process to provide refrigeration.

Stream 116 of carbon dioxide liquid from the first separator 112 issplit into two streams. The first stream 154 is pumped to the final CO2pressure of 110 bar (11 MPa) in pump 156 and fed as stream 158 to themain heat exchanger 162 where it is vaporized and warmed by indirectheat exchange to produce stream 160 of carbon dioxide gas. The secondstream 117 is reduced in pressure in valve 118 to produce a stream 120of reduced pressure carbon dioxide liquid at about 18 bar (1.8 MPa)which is then fed to the main heat exchanger 162 where it is vaporizedand warmed to produce a stream 122 of carbon dioxide gas.

Stream 132 of carbon dioxide liquid is warmed to produced a stream 134of warmed carbon dioxide liquid which is then reduced in pressure invalve 136 to produce a stream 138 of reduced pressure carbon dioxideliquid at about 8 bar (0.8 MPa). Stream 138 is fed to the main heatexchanger 162 where it is vaporized and warmed to produce a stream 140of warmed carbon dioxide gas.

Stream 140 is compressed in a first CO2 product compressor 142 toproduce a stream 144 of compressed carbon dioxide gas at about 18 bar(1.8 MPa). Streams 144 and 122 of carbon dioxide gas are combined, andthe combined stream 146 is compressed in a second CO2 product compressor148 to produce a stream 150 of compressed carbon dioxide gas at about110 bar (11 MPa). Streams 150 and 160 are combined to form productstream 152.

The refrigeration duty in the CPU is provided primarily by indirect heatexchange with carbon dioxide liquids produced from the flue gas, withabout 6% of the cooling duty being provided by indirect heat exchangewith stream 128 comprising non-condensible gases.

The ASU exemplified in FIG. 1 has three columns, viz. a higher pressurecolumn 1050, an intermediate pressure column 1022, and a lower pressurecolumn 1110. A stream 1000 of ambient air is compressed in a first part1002 of a main air compressor to produce a stream 1004 of compressed airwhich is then cooled and cleaned of impurities in front-end purificationunit 1006 to produce a stream 1008 which is split into two parts,streams 1010 and 1040. The first part 1010 flows directly to the mainheat exchanger 1012, and the second part 1040 is further compressed in asecond part 1042 of the main air compressor and is then fed as stream1044 to the main exchanger.

Intermediate pressure stream 1010 is cooled to an intermediatetemperature and split into two parts. The first part 1014 is expanded inexpander 1016 to form expanded stream 1018 and fed to the lower pressurecolumn 1110. The second part is further cooled in the main exchanger1012 and fed as stream 1020 to the intermediate pressure column 1022.

The higher pressure stream 1044 is cooled in the main exchanger 1012 toclose to its dew point to form stream 1046 which is then split into twofurther streams, 1048 and 1060. Stream 1048 feeds the higher pressurecolumn 1050. Stream 1060 is condensed in an oxygen product reboiler 1062to form stream 1064 which is split to form streams 1068 and 1072. Stream1068 is reduced in pressure in valve 1070 and fed to the higher pressurecolumn 1050. Stream 1072 is reduced in pressure in valve 1074 to formstream 1076 which is fed to the intermediate pressure column 1022.

In both the higher pressure column 1050 and the intermediate pressurecolumn 1022, a vapour air feed is separated into a nitrogen-enrichedoverhead vapour and an oxygen-enriched bottoms liquid.

A stream 1052 of oxygen-rich liquid from the higher pressure column 1050is reduced in pressure in valve 1054 and then fed to the bottom of theintermediate pressure column as stream 1056. A stream 1024 ofoxygen-rich liquid is removed from the bottom of the intermediatepressure column 1022, reduced in pressure in valve 1026 and fed asstream 1028 to a condenser 1030 for the intermediate pressure column1022. The reduced pressure oxygen-rich liquid is partially boiled byindirect heat exchange to produce liquid and vapour parts which are fedas streams 1032 and 1034 respectively to the lower pressure column 1110.

Nitrogen-rich overhead vapour from the top of the higher pressure columnis condensed by indirect heat exchange in the main reboiler-condenser1120, and part of the resulting liquid is returned to the higherpressure column 1050 as reflux. The rest of the nitrogen-rich liquid(stream 1100) is subcooled by indirect heat exchange to produce stream1102, reduced in pressure in valve 1104 and fed as stream 1106 to thetop of the lower pressure column 1110 as reflux.

A part of the nitrogen-rich overhead vapour from the top of theintermediate pressure column 1022 is fed as stream 1080 to the main heatexchanger 1012 where it is is warmed to produce a stream 200 of gaseousnitrogen. The rest of this vapour is condensed in reboiler-condenser1030. Part of the resulting liquid is returned to the intermediatepressure column 1022 as reflux. The remaining part, stream 1084, isreduced in pressure in valve 1086 and fed as stream 1088 to the top ofthe lower pressure column 1110.

A liquid stream 1090 having a composition similar to air is taken froman intermediate location in the higher pressure column 1050, subcooledby indirect heat exchange to form stream 1092, reduced in pressure invalve 1094 to form stream 1096 and fed to an intermediate location inthe lower pressure column 1110.

The lower pressure column 1110 separates the feeds into nitrogenoverhead vapour and oxygen bottoms liquid. Liquid oxygen is withdrawn asstream 1122 from the bottom of the lower pressure column 1110, adjustedin pressure in valve 1124 to form stream 1126 and boiled in oxygenproduct reboiler 1062. A liquid oxygen product stream 1128 may be takenfrom this reboiler, but most of the oxygen product is taken as vapour instream 1130, and warmed in the main exchanger 1012 to form stream 1132of gaseous oxygen. Stream 1132 can be used for combustion of coal in theboiler.

A stream 1112 of nitrogen is taken from the top of the lower pressurecolumn 1112, warmed by indirect heat exchange, first in the subcooler toproduce stream 1114 and then in the main heat exchanger 1012, to producestream 1116 which is then used in the front-end cooling and purificationsystem 1006 before being vented to the atmosphere as stream 1118.

A report by Allam & Spilsbury entitled “A study of the extraction of CO2from the flue gas of a 500 MW pulverized coal fired boiler” (EnergyConyers. Mgmt. Vol. 33; No. 5-8; pp 373-378, 1992) discloses an oxyfuelcombustion process for power generation in which oxygen is supplied froman integrated ASU, and in which carbon dioxide from the flue gas ispurified by low temperature distillation under elevated pressure.Refrigeration duty for the process is provided by indirect heat exchangewith individually expanded streams containing non-condensible componentsfrom the flue gas itself, and oxygen gas and waste nitrogen gas from theASU. All of the carbon dioxide product is in liquid form.

EP 0 965 564 A discloses a process for the low temperature purificationof carbon dioxide by distillation under elevated pressure. Refrigerationduty for the process is provided by evaporation of liquid ammonia in aclosed loop refrigeration cycle and a small portion of the carbondioxide liquid produced in the process.

WO 2010/017968 A discloses a process for the cryogenic distillation ofair to supply oxygen to an oxyfuel power plant. Refrigeration duty forthe process is provided by indirect heat exchange with product streamsfrom the distillation, including a nitrogen gas stream which is takenfrom the higher pressure column, and is warmed and expanded. A keyfeature of this process is that additional pressurized nitrogen iswarmed above ambient temperature, expanded, re-warmed and re-expanded torecover power.

FR 2 934 170 A discloses a CPU process in which refrigeration duty isprovided primarily by evaporating carbon dioxide product liquid atdifferent pressure levels.

In an oxyfuel power plant with CO2 capture, both the ASU and CPU consumepower and reduce the power available for export. For example, in areport by Fu & Gundersen entitled “Heat integration of an oxy-combustionprocess for coal-fired power plants with CO2 capture by pinch analysis”(Chemical Engineering Transactions, Vol. 21, 2010, pp 181-186), there isdisclosed an exergy analysis for an oxy-combustion process for asupercritical pulverized coal power plant with CO2 capture. The resultsindicate that the compression processes in the ASU and CPU areresponsible for the largest exergy losses related to CO2 capture. Thereport speculates that these losses can be reduced by by heatintegration between the ASU and CPU. However, no further details of howthe heat integration might be achieved are provided.

BRIEF SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method ofpurifying crude carbon dioxide gas in which the total parasitic powerconsumption of the CPU can be reduced.

It is an object of preferred embodiments of the present invention toprovide a method of generating power using an oxyfuel combustion processwith CO2 capture, in which the total parasitic energy consumption of theASU and CPU is reduced.

It is an object of preferred embodiments of the present invention toprovide a method of purifying crude carbon dioxide gas, or of generatingpower, in which carbon dioxide liquid may be withdrawn as a product andoptionally stored or pumped to line pressure prior to vaporisation.

It is an object of preferred embodiments of the present invention toprovide a method of purifying crude carbon dioxide gas, or of generatingpower, in which the energy required to compress gaseous carbon dioxideproduct is reduced.

The Inventor has discovered that the total parasitic power consumptionof a carbon dioxide purification plant purifying a crude carbon dioxidegas can be reduced significantly provided that the refrigeration dutyfor cooling the feed to the plant is provided in part by evaporating atleast one first liquid refrigerant (e.g. carbon dioxide liquid derivedfrom the feed and/or recirculating ammonia), and in another part byheating (without evaporation) at least one further refrigerant fluid,preferably a fluid that is independent of, i.e. not derived from, thecrude carbon dioxide gas.

Thus, according to a first aspect of the present invention, there isprovided a method of purifying crude carbon dioxide gas containing atleast one non-condensible gas contaminant, under elevated pressure in aCPU, said method comprising:

feeding crude carbon dioxide gas to the CPU;

cooling and condensing carbon dioxide gas from the crude carbon dioxidegas; and

separating condensed carbon dioxide gas from said non-condensible gascontaminant(s) to produce at least one carbon dioxide liquid and a tailgas comprising said non-condensible gas contaminant(s),

wherein the method requires refrigeration duty for cooling andcondensing carbon dioxide gas, said refrigeration duty being provided inat least a first part by indirect heat exchange against at least oneliquid first refrigerant thereby evaporating said first refrigerant(s),and a second part by indirect heat exchange against sensible heat energyalone of at least one second refrigerant.

The method has particular application where the CPU is purifying fluegas from an oxyfuel combustion process, or where the CPU is integratedwith a cryogenic ASU. In preferred embodiments, the CPU is purifyingflue gas from an oxyfuel combustion process to which oxygen is suppliedfrom an ASU that is integrated with the CPU.

In an oxyfuel power plant that includes both an ASU for supplying oxygenfor the combustion, and a CPU for purifying the flue gas, the powerconsumption of the combined system may be reduced by providing part ofthe refrigeration duty for the CPU by expanding nitrogen from the ASU.Even though the power consumption of the CPU is not reduced by as muchas the energy required to compress nitrogen from atmospheric pressure toits supply pressure, the power of the carbon dioxide compression can bestill be reduced by more than the ASU power increases when it producesthe pressurized nitrogen, resulting in a net power reduction. Thus, inpreferred embodiments, the CPU and ASU are thermally integrated.

The second refrigerant is preferably nitrogen gas imported from the ASUor carbon dioxide liquid exported from the CPU, subcooled in the ASU andimported back to the CPU. In such embodiments, the total energyconsumption of the ASU and CPU can be reduced significantly.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts flowsheets for a prior art CPU and ASU known for use withan oxyfuel combustion process.

FIG. 2 depicts a CPU flowsheet according to a first embodiment of thepresent invention.

FIG. 2 a depicts the CPU flowsheet of FIG. 2 in which the secondrefrigerant circulates in a closed loop refrigeration cycle.

FIG. 2 b depicts the CPU flowsheet of FIG. 2 in which the secondrefrigerant is expanded nitrogen gas imported into the CPU at elevatedpressure from an ASU.

FIG. 3 depicts a CPU flowsheet according to a second embodiment of thepresent invention.

FIG. 3 a depicts the CPU flowsheet of FIG. 3 in which the secondrefrigerant is expanded nitrogen gas imported into the CPU at elevatedpressure from an ASU.

FIG. 4 depicts a CPU flowsheet according to a third embodiments of thepresent invention.

FIG. 4 a depicts the CPU flowsheet of FIG. 4 in which the secondrefrigerant is expanded nitrogen gas imported into the CPU from the ASU.

FIG. 5 depicts a CPU flowsheet according to a fourth embodiment of thepresent invention.

FIG. 5 a depicts the CPU flowsheet of FIG. 5 in which the secondrefrigerant is subcooled carbon dioxide liquid produced by the CPU andsubcooled in the ASU.

DETAILED DESCRIPTION OF THE INVENTION

The invention concerns a method of purifying dry crude carbon dioxidegas containing at least one non-condensible gas contaminant, underelevated pressure in a CPU. The method comprises feeding crude carbondioxide gas to the CPU; cooling and condensing carbon dioxide gas fromthe crude carbon dioxide gas; and separating condensed carbon dioxidegas from said non-condensible gas contaminant(s) to produce at least onecarbon dioxide liquid and a tail gas comprising the non-condensible gascontaminant(s). The method requires refrigeration duty for cooling andcondensing carbon dioxide gas. The refrigeration duty is provided in atleast a first part by indirect heat exchange against at least one liquidfirst refrigerant thereby evaporating said first refrigerant(s), and asecond part by indirect heat exchange against sensible heat energy aloneof at least one second refrigerant. It should be noted that, while thesecond refrigerant gives up sensible heat alone, the crude carbondioxide gas still condenses.

The carbon dioxide purification process in the CPU takes place atsub-ambient temperature, for example at a temperature below 0° C., e.g.from about −20° C. to about −60° C.

The crude carbon dioxide gas is dry, i.e. the vapour pressure of waterin the crude carbon dioxide gas is less than the vapour pressure of iceat the lowest temperature. For example, there should be less than 0.4ppm water if the crude carbon dioxide gas is at −60° C. and 30 bar, orless than 0.2 ppm water if the crude carbon dioxide gas is at −60° C.and 60 bar, or less than 1.4 ppm water if the crude carbon dioxide gasis at −50° C. and 30 bar.

The crude carbon dioxide gas to be purified in the CPU usually containsat least 40% carbon dioxide, e.g. from about 40% to about 90%, or fromabout 60% to about 90%, or from about 65% to about 85%, carbon dioxide.Flue gas from an oxyfuel combustion process typically contains suchamounts of carbon dioxide and the method has particular application inthe purification of flue gas generated by oxyfuel combustion of a fuelselected from the group consisting of carbonaceous fuel, such as coal;hydrocarbonaceous fuel, such as methane or natural gas; and biomass.

The term “elevated pressure” is intended to mean a pressure that issignificantly greater than atmospheric pressure. For example, the termis intended to exclude minor elevations in pressure over atmosphericpressure, such as those elevations provided by a blower or fan in orderto force a gas through apparatus operating at about atmosphericpressure. Such minor pressure elevations are considered to beinsignificant in the context of the present invention.

The elevated pressure in the CPU is usually at least 2 bar (0.2 MPa),e.g. at least 3 bar (0.3 MPa), or at least 5 bar (0.5 MPa). The elevatedpressure is usually no more than about 100 bar (10 MPa) and preferablyno more than about 50 bar (5 MPa). The elevated pressure may be fromabout 10 bar to about 50 bar (0.5 MPa to 5 MPa), or from about 20 bar toabout 40 bar (1 MPa to 4 MPa), e.g. about 30 bar (3 MPa).

The CPU comprises at least one heat exchanger within which the carbondioxide gas is cooled and condensed by indirect heat exchange againstthe refrigerants.

The crude carbon dioxide gas may be purified in the CPU by distillationto produce carbon dioxide bottoms liquid and an overhead vapour (or tailgas) comprising the non-condensible gas contaminant(s). In suchembodiments, the CPU comprises a distillation column system.

However, in preferred embodiments, the crude carbon dioxide gas ispurified by cooling and condensing the carbon dioxide gas by indirectheat exchange, thereby leaving the non-condensible gas contaminant(s) asvapour. The condensed carbon dioxide gas is then separated from thenon-condensible gas contaminant(s) in a phase separator to produce acarbon dioxide liquid and an overhead vapour. If this overhead vapourshould contain a sufficiently low concentration of carbon dioxide gas,then it can provide the tail gas. On the other hand, if the overheadvapour should still contain significant quantities of carbon dioxide,then the overhead vapour may be further cooled to condense furthercarbon dioxide gas by indirect heat exchange, thereby leaving thenon-condensible contaminant(s) as vapour. The condensed further carbondioxide gas is then separated from the non-condensible gas contaminantsin a further phase separator to produce a further carbon dioxide liquidand a further overhead vapour (or tail gas). Additional stages ofcooling and phase separation can be used as required.

The refrigeration duty of the purification method is the heat that needsto be removed from the crude carbon dioxide gas to cool and condensecarbon dioxide in the gas. In the present invention, this duty isprovided in at least a first part by indirect heat exchange against atleast one liquid first refrigerant thereby evaporating the firstrefrigerant(s), and a second part by indirect heat exchange againstsensible heat energy alone of a second refrigerant. The secondrefrigerant can also remove latent heat from the crude carbon dioxidegas feed as the feed will be condensing as it is being cooled once it isbelow its dew point.

The terms “first part” and “second part” in the context of therefrigeration duty are intended as general terms meaning first andsecond portions respectively of the total refrigeration duty requiredfor cooling and condensing the carbon dioxide. These terms do notnecessarily refer to the duty required above and below the carbondioxide dew point.

The indirect heat exchange usually takes place within the main heatexchanger within the CPU.

Evaporation of the liquid first refrigerant(s) involves an exchange ofthermal energy in the form of latent heat energy to the liquid firstrefrigerants(s) from the crude carbon dioxide gas. In embodimentsinvolving a liquid first refrigerant under supercritical conditions,that liquid first refrigerant “pseudo-evaporates”. The term“evaporation” is intended to embrace pseudoevaporation in embodimentswhere the liquid first refrigerant is a supercritical liquid.

“Pseudoevaporation” is the rapid reduction in density with increasingtemperature that occurs (with a relatively large heat input in a narrowtemperature range) in a supercritical fluid as it is heated from below apseudo-saturation temperature (point of inflection on the temperatureversus enthalpy curve) to above it. Therefore, the supercritical fluidgoes from a denser, liquid-like state to a less dense, gas-like stateover a temperature range but without a definitive phase change.

Sensible heat energy may also be transferred prior to, and/or after,transfer of the latent heat energy resulting, for example, in warming ofthe resultant vaporized first refrigerant.

Sensible heat energy is the energy exchanged by a thermodynamic systemthat has, as its sole effect, a change of temperature, i.e. there is nochange in the phase of the system. The second part of the refrigerationduty in the present invention is provided by indirect heat exchange ofsensible heat energy alone. Thus, where the second refrigerant is asubcritical liquid, there is no evaporation of the second refrigerantwhen providing the second part of the refrigeration duty.

The first part of the refrigeration duty may be provided by a singleliquid first refrigerant, or by more than one liquid first refrigerant.

The liquid first refrigerant or, where there is more than one, at leastone of the liquid refrigerants may be an external refrigerant, that is aliquid refrigerant that is independent of, i.e. not derived from, thecrude carbon dioxide gas. A suitable example of such a refrigerant isliquid ammonia. In preferred embodiments, such a refrigerant circulatesin a closed loop refrigeration cycle.

The liquid first refrigerant or, where there is more than one, at leastone of the liquid refrigerants may be liquefied carbon dioxide, e.g. thecarbon dioxide liquid, or where there is more than one, at least one(and preferably each) of the carbon dioxide liquids produced in the CPU.In embodiments in which more than one carbon dioxide liquid is producedin the CPU, at least a portion of each of the carbon dioxide liquids isusually used to provide the first part of the refrigeration duty as theliquid first refrigerants.

The or each liquid first refrigerant is typically evaporated by theindirect heat exchange against the cooling and condensing crude carbondioxide gas. Where the liquid first refrigerant is supercritical, ittypically “pseudo-evaporates”.

In preferred embodiments, a further part of the refrigeration duty isprovided by indirect heat exchange against the tail gas. The furtherpart is usually no more than 10%, preferably from about 5% to 7%, e.g.about 6%, of the refrigeration duty. The remainder of the refrigerationduty is usually composed of the first and second parts.

Providing refrigeration by expansion of a gas in a reverse Brayton cycleis less efficient that evaporating liquid in a reverse Rankine cycle inthe temperature range encountered in a CPU. Therefore, the Inventor hasdetermined that the second refrigerant is preferably used as cold aspossible so that the reverse Brayton cycle is as efficient as possible,thereby maximizing the overall benefit. In this connection, the secondrefrigerant preferably has a temperature that is sufficiently low tocool the crude carbon dioxide gas to below the CO2 dew pointtemperature, and preferably to close to the CO2 triple pointtemperature. For example, the crude carbon dioxide gas is cooled to atemperature from about −56° C. to about 10° C. As would be readilyappreciated by the skilled person, the CO2 dew point temperature dependson the composition and pressure of the gas. In addition, the skilledperson would appreciate that the temperature of the second refrigerantcould be lower than −56° C. provided that the heat exchanger is designedto avoid freezing on its surface, for example by using co-current flowat the cold end.

The refrigeration duty generally has a colder part that both cools andcondenses said crude carbon dioxide gas, and a warmer part that coolsthe carbon dioxide gas with no condensation. In these embodiments, thepoint at which the colder part of the refrigeration duty becomes thewarmer part is the CO2 dew point of the crude carbon dioxide gas. Sinceit is preferably used as cold as possible, the second refrigerantusually provides at least a portion of the colder part of therefrigeration duty.

Since a reverse Brayton cycle tends to be less efficient that a reverseRankine cycle for providing refrigeration, the second refrigerant ispreferably used in a quantity that is no more than that sufficient tooptimize power consumption. Generally, the second refrigerant providesno more than 30%, preferably no more than 25%, of the refrigerationduty. For example, for nitrogen extraction from an ASU, typically amolar nitrogen flow between 0.5 and 1.5 times the oxygen flow optimizespower consumption and this provides a net cooling duty below the feeddew point between about 7% and 21% of the total feed cooling duty in theCPU. The second refrigerant usually provides more than 1% of therefrigeration duty.

The second refrigerant is preferably independent of, i.e. not derivedfrom, the crude carbon dioxide gas. The second refrigerant may bederived from said crude carbon dioxide gas and exported from the CPU forcooling. In some preferred embodiments, the second refrigerant isimported into the CPU.

The second refrigerant may be a liquid, preferably a subcritical liquid(which is not vaporized during heat exchange). However, in preferredembodiments, the second refrigerant is a gas, preferably nitrogen gas.

The CPU is preferably integrated with a cryogenic ASU and the secondrefrigerant is nitrogen gas imported from the ASU.

In preferred embodiments, the method comprises importing a pressurizedgas into the CPU; and expanding the pressurized gas in said CPU afteroptionally cooling said gas, to produce the second refrigerant. In theseembodiments, the method may comprise separating compressed air bycryogenic distillation in an ASU to produce nitrogen gas under pressureand gaseous oxygen; importing at least a portion of the nitrogen gasinto the CPU; cooling the imported nitrogen gas by indirect heatexchange to form cooled nitrogen gas; expanding the cooled nitrogen gasin the CPU to produce expanded nitrogen gas; and using said expandednitrogen gas as said second refrigerant to produce warmed nitrogen gas.The cooled nitrogen gas may be expanded in a first expander, warmed andthen further expanded in a second expander, and optionally re-warmed.

The temperature of the imported nitrogen gas will vary according to theambient conditions as the ASU design. For example, the imported nitrogengas may have a temperature in the range from about 0° C. to about 50° C.

The imported nitrogen gas may then be cooled by indirect heat exchangeagainst the first liquid refrigerant(s), the second refrigerant and/oranother stream such as the tail gas comprising non-condensible gases toa temperature in the range from about −100° C. to about 10° C. andpreferably to below the CO2 dew point of the crude carbon dioxide gas.

The cooled nitrogen may be expanded over a modest pressure ratio, sayless 5 and preferably less than 3. For example, the cooled nitrogen maybe expanded from a first pressure in the range from about 1.5 to about 8bar to a second pressure in the range from about 1 bar to about 4 bar,e.g. from about 5.5 bar to about 1.1 bar.

A portion of the carbon dioxide liquid may be removed as a liquidproduct, and optionally pumped and warmed to ambient temperature.

Since the air is separated in the ASU at cryogenic temperatures, waterand carbon dioxide are removed before the air is cooled and fed to thedistillation column system of the ASU. The ASU may comprise a singledistillation column operating at elevated pressure, or more than onedistillation column, each column operating at different elevatedpressures. In preferred embodiments, the ASU comprises either a dualcolumn arrangement comprising a higher pressure column and a lowerpressure column in which the columns are thermally integrated by areboiler/condenser, or a tri-column arrangement comprising a higherpressure column, an intermediate pressure column and a lower pressurecolumn in which the higher pressure column is thermally integrated withthe lower pressure column by a first reboiler/condenser, and theintermediate pressure column is thermally integrated with the lowerpressure column via a second reboiler condenser. The operating pressureof the higher pressure column is usually from about 3 bar to about 12bar (0.3 to 1.2 MPa). The operating pressure of the lower pressurecolumn is usually from about 1.1 bar to about 5 bar (0.11 to 0.5 MPa).The operating pressure of an intermediate pressure column is usuallyfrom about (1.8 bar to about 8 bar (0.18 to 0.8 MPa).

In other preferred embodiments, the method comprises importing thesecond refrigerant into the CPU and using the second refrigerantdirectly, i.e. without further processing, to provide the second part ofthe refrigeration duty. In these embodiments, the method may compriseseparating compressed air by cryogenic distillation in an ASU to producenitrogen gas under pressure and gaseous oxygen; expanding at least aportion of the nitrogen gas to produce expanded nitrogen gas; importingthe expanded nitrogen gas into the CPU; and using said expanded nitrogengas as said second refrigerant to produce warmed nitrogen gas. Water andcarbon dioxide are usually removed from compressed air, typically in apurification unit having at least one sorbent bed, to produce thecompressed air feed for the ASU. In such embodiments, the sorbent bed(s)may be regenerated using at least a portion of the warmed nitrogen gas.Expanded nitrogen gas could also be used to regenerate CPU feed drierbeds.

The second refrigerant may be liquefied carbon dioxide, for example, atleast a portion of the carbon dioxide liquid(s) produced in the CPU. Themethod of these embodiments comprises cooling and partially condensingcarbon dioxide gas in the crude carbon dioxide gas; separating thecondensed carbon dioxide gas from said non-condensible gascontaminant(s) in a first phase separator to produce a first carbondioxide liquid and a first overhead vapour comprising saidnon-condensible gas contaminant(s); dividing said first carbon dioxideliquid into three portions; pumping a first portion of said first carbondioxide liquid to produce a pumped first portion; reducing the pressureof a second portion of said first carbon dioxide liquid to produce areduced pressure second portion; pumping a third portion of said firstcarbon dioxide liquid to produce a pumped third portion, and cooling thepumped third portion to produce a cooled third portion; cooling andcondensing carbon dioxide gas in said first overhead vapour; separatingthe condensed carbon dioxide gas from said non-condensible gascontaminant(s) in a second phase separator to produce a second carbondioxide liquid and said tail gas comprising said non-condensible gascontaminant(s); warming said tail gas by indirect heat exchange toproduce warmed tail gas; and reducing the pressure of said second carbondioxide liquid after optionally warming said liquid, to produce reducedpressure second carbon dioxide liquid. In these embodiments, the liquidfirst refrigerant(s) providing the first part of the refrigeration dutyare the pumped first portion of first carbon dioxide liquid, the reducedpressure second portion of first carbon dioxide liquid and the reducedpressure second carbon dioxide liquid, and the second refrigerant is thecooled third portion of the first carbon dioxide liquid. The pumpedthird portion of the first carbon dioxide liquid may be cooled byindirect heat exchange against expanded nitrogen gas in an integratedASU.

In other embodiments, the second refrigerant may circulate in a closedloop refrigeration cycle within the CPU. In such embodiments, where thesecond refrigerant is nitrogen gas, make up nitrogen gas may be importedfrom an integrated ASU as required.

Since providing refrigeration by expansion of a gas in a reverse Braytoncycle is less efficient that evaporating liquid in a reverse Rankinecycle in the temperature range encountered in a CPU, the refrigerant ofchoice for an independent CPU is the carbon dioxide itself. Only atcolder temperatures would the reverse Brayton cycle become moreefficient.

Therefore, to maximize the overall benefit, the refrigeration providedto the CPU by expanding nitrogen from the ASU should be as cold aspossible (so that the reverse Brayton cycle is as efficient as possible)and use only the quantity of nitrogen necessary to optimize the ASUdesign (to minimize the impact of the lower efficiency refrigerationcycle). This refrigeration is particularly suited to the colder part ofthe feed condensation. The provision of external refrigeration allowspart of the carbon dioxide to be produced as liquid that may be stored,pumped to the final product pressure (and optionally reheated), or as acold gas that may be compressed with reduced power input. The balance ofthe cooling of the CPU feed is provided by evaporating carbon dioxide inan open cycle or reverse Rankine cycle.

The liquid first refrigerant(s) is usually at a temperature in the rangefrom about −56° C. to about 10° C., preferably from about −40° C. toabout −10° C. The second refrigerant is usually at a temperature in therange from −100° C. to 10° C., preferably from about −70° C. to about−10° C. Suitable pressures for the first and second refrigerants willdepend on the fluid used. For carbon dioxide as the liquid firstrefrigerant, the pressure is usually in the range from about 5.2 bar to42 bar, and for nitrogen as the second refrigerant, the pressure isusually in the range from about 1 to about 4 bar.

One advantage of preferred embodiments of the present invention is thatat least a portion of the carbon dioxide liquid(s) may be removed fromthe CPU and stored as a liquid. In the prior art depicted in FIG. 1,almost the entire inventory of carbon dioxide liquid has to be used toprovide refrigeration as typically no more than 5% of the carbon dioxideliquid produced may be withdrawn as a liquid product.

In contrast, the present invention enables removal of significantly moreof the carbon dioxide liquid from the CPU as product for storage. Inthis connection, without an external refrigerant, up to about 50%, e.g.from more than 5% to about 50% or from about 10% to about 30%, of thecarbon dioxide liquid can be removed as a liquid product, depending onthe proportion of the refrigeration duty provided by the carbon dioxideliquid. If the evaporating refrigerant is from an external source, e.g.an external ammonia refrigeration cycle, then all of the carbon dioxideliquid could be withdrawn as product. The removed liquid may be pumpedto line pressure, for example at least 50 bar (5 MPa), e.g. from about100 bar to about 200 bar (10 MPa to 20 MPa), and optionally heated toambient temperature.

The purification method can be used in a method for generating powerusing an oxyfuel combustion process. In oxyfuel combustion, the fuel iscombusted in an oxygen-rich atmosphere comprising at least 20 wt %oxygen and recycled flue gas from the combustion process to moderate thetemperature of combustion and control heat flux. Oxygen is usuallysupplied as pure oxygen, or as an oxygen-rich gas, e.g. a gas comprisingat least 80% 02, and is usually supplied from a cryogenic ASU.

The method may be used to purify crude carbon dioxide gas having a flowrate from 200 kmol/h to 40,000 kmol/h which flow rates are typical fornet flue gas generated in a “standard” single unit oxyfuel combustionprocess. Higher flows are possible, e.g. from about 40000 kmol/h toabout 150000 kmol/h, for example if the crude gas is provided from verylarge power stations, or from multiple “standard” oxyfuel combustionunits.

Flue gas from an oxyfuel combustion process usually contains carbondioxide as the major component, together with SOx, NOx and thenon-condensable gases O2, N2, Ar, Kr and Xe. SOx is produced by thecombustion of elemental sulfur and/or sulfur-containing compoundspresent in the fuel. 02 is present in the flue gas from excess O2 usedin the combustion and from air ingress into the combustion unit which isalso responsible for the presence of N2, Ar, Kr and Xe in the flue gas.NOx is produced by reaction N2 with O2 in the combustion unit.

Further components in the flue gas include solid particulates such asfly ash and soot; water; CO; HCl; CS2; H2S; HCN; HF; volatile organiccompounds (VOCs) such as CHCl3; metals including mercury, arsenic, iron,nickel, tin, lead, cadmium, vanadium, molybdenum and selenium; andcompounds of these metals.

Flue gas from the combustor is typically washed with water to removeparticulates (such as soot and/or fly ash) and water soluble components(such as HF, HCl and/or SO3). Additionally, the flue gas may befiltered, using equipment such as a baghouse or electrostaticprecipitator, to enhance particulate removal. Flue gas may also bedesulfurized, e.g. in a flue gas desulfurization unit (FGD), beforebeing fed to the CPU for purification.

Since the flue gas is typically at atmospheric pressure, it is thencompressed after washing to the elevated pressure to form the carbondioxide feed gas to be purified by the method. However, if the feed gasoriginates from a source, such as a pressurized oxyfuel combustionsystem, that is already at the required elevated pressure, thencompression is not required.

SOx and/or NOx may be removed as sulfuric acid and nitric acidrespectively by providing sufficient “hold up” at elevated pressure inthe presence of water and oxygen.

The power generation methods according to the present invention comprisecombusting a fuel selected from the group consisting of carbonaceousfuels, hydrocarbonaceous fuels and biomass, in an oxygen-rich atmospherewithin an oxyfuel combustion unit to produce heat and flue gas;recovering at least a portion of the heat to generate the power;dividing the flue gas after optionally desulfurizing the flue gas, intorecycle flue gas and net flue gas; recycling the recycle flue gas to theoxyfuel combustion unit; and compressing and drying the net flue gas toproduce dry flue gas under elevated pressure containing at least onenon-condensible contaminant.

In a first embodiment, the power generation method comprises cooling andcondensing carbon dioxide gas from said dry flue gas, and separatingcondensed carbon dioxide gas from said non-condensible gascontaminant(s) to produce at least one carbon dioxide liquid and a tailgas comprising said non-condensible gas contaminant(s); separatingcompressed air by cryogenic distillation in an ASU to produce nitrogengas under elevated pressure and gaseous oxygen; feeding at least aportion of the gaseous oxygen to the oxyfuel combustion unit; andfeeding at least a portion of the nitrogen gas to the CPU wherein thenitrogen gas is cooled by indirect heat exchange to produce coolednitrogen gas which is then expanded to produce expanded nitrogen gas.The refrigeration duty in the CPU is provided in at least a first partby indirect heat exchange against at least a portion of the carbondioxide liquid(s) thereby evaporating the liquid(s), and a second partby indirect heat exchange of sensible heat alone against the expandednitrogen gas. The cooled nitrogen gas may be expanded in a firstexpander, warmed and then further expanded in a second expander toproduce the expanded nitrogen gas.

In a second embodiment, the power generation method comprises feedingthe dry flue gas to a CPU wherein carbon dioxide gas from said dry fluegas is cooled and condensed, and separated from said non-condensible gascontaminant(s) to produce at least one carbon dioxide liquid and a tailgas comprising said non-condensible gas contaminants; separatingcompressed air by cryogenic distillation in an ASU to produce nitrogengas under elevated pressure and gaseous oxygen; feeding at least aportion of the gaseous oxygen to the oxyfuel combustion unit; expandingin the ASU at least a portion of the nitrogen gas to produce expandednitrogen gas; and feeding the expanded nitrogen gas to the CPU. Therefrigeration duty in the CPU is provided in at least a first part byindirect heat exchange against at least a portion of said carbon dioxideliquid(s) thereby evaporating said liquid(s), and a second part byindirect heat exchange against sensible heat energy alone of saidexpanded nitrogen gas.

The imported nitrogen gas may be cooled to below the dew point of thecarbon dioxide in the feed, e.g. to a temperature in the range fromabout −40° C. to about 10° C.

The cooled nitrogen gas may be expanded over a modest pressure ratio,for example less than 5, and preferably less than 3.

A portion of the carbon dioxide liquid may be removed as a productliquid, and optionally pumped and warmed to ambient temperature.

In a third embodiment, the power generation method comprises feeding thedry flue gas to a CPU for purification, said purification comprisingcooling and condensing carbon dioxide gas in the dry flue gas;separating the condensed carbon dioxide gas from said non-condensiblegas contaminant(s) in a first phase separator to produce a first carbondioxide liquid and a first overhead vapour comprising saidnon-condensible gas contaminant(s); dividing said first carbon dioxideliquid into three portions; pumping a first portion of said first carbondioxide liquid to produce a pumped first portion; reducing the pressureof a second portion of said first carbon dioxide liquid to produce areduced pressure second portion; pumping a third portion of said firstcarbon dioxide liquid to produce a pumped third portion, and cooling thepumped third portion to produce a cooled third portion; cooling andcondensing carbon dioxide gas in said first overhead vapour; separatingthe condensed carbon dioxide gas from said non-condensible gascontaminant(s) in a second phase separator to produce a second carbondioxide liquid and said tail gas comprising said non-condensible gascontaminant(s); warming said tail gas by indirect heat exchange toproduce warmed tail gas; and reducing the pressure of said second carbondioxide liquid after optionally warming said liquid, to produce reducedpressure second carbon dioxide liquid. The method comprises alsoseparating compressed air by cryogenic distillation in an ASU to producenitrogen gas under elevated pressure and gaseous oxygen; feeding atleast a portion of the gaseous oxygen to said oxyfuel combustion unit;and expanding in the ASU at least a portion of the nitrogen gas toproduce expanded nitrogen gas. The refrigeration duty in the CPU isprovided in at least a first part by indirect heat exchange with thepumped first portion of first carbon dioxide liquid, the reducedpressure second portion of first carbon dioxide liquid and the reducedpressure second carbon dioxide liquid, and a second part by indirectheat exchange of with sensible heat energy alone of the cooled thirdportion of the first carbon dioxide liquid. The pumped third portion ofthe first carbon dioxide liquid may be cooled in the ASU by indirectheat exchange against the expanded nitrogen gas.

FIG. 2 depicts the CPU process of FIG. 1 to which an expander 204 hasbeen added. An external vapour stream 200 is cooled to an intermediatetemperature in the main heat exchanger 162 to produce cooled stream 202which is expanded in expander 204 to produce expanded stream 206. Theexpanded stream 206 is warmed in the main heat exchanger 162 to producean exhaust stream 208 of warmed refrigerant gas which may be vented toatmosphere, or used for some other purpose, for example evaporativechilling of water.

FIG. 2 a depicts the same process as FIG. 2 in which the exhaust stream208 is recirculated in a compressor 210 where it is compressed to formstream 200 in a closed loop refrigeration cycle. Only a very smallmake-up stream is required to replace any losses from the system.

FIG. 2 b depicts the ASU of FIG. 1 integrated with the CPU of FIG. 2through a pressurised nitrogen stream 200 from the intermediate pressurecolumn 1022 of the ASU.

FIG. 3 depicts the process of FIG. 2 to which a second expander 302 hasbeen added. The exhaust stream 206 of the first expander 204 is warmedin the main exchanger 162 to produce a warmed stream 300 which is thenfurther expanded in the second expander 302 to form a further expandedstream 304. Stream 304 is warmed in the main heat exchanger 162 toproduce exhaust stream 208 which is then and vented, recycled or used inanother process. This process is useful if the pressure of the secondrefrigerant is higher than about 3 bar (0.3 MPa), as the refrigerationfrom a single expander would otherwise be provided less efficiently overtoo wide a temperature range.

FIG. 3 a depicts the process of FIG. 3 integrated with an alternativedual column ASU process in which nitrogen is taken from the higherpressure column 1050 operating at around 4 bar (0.4 MPa). Since theintermediate pressure column 1022 has been omitted, the entire air feedgoes to the higher pressure column 1050; stream 1056 of oxygen-richbottoms liquid is fed from the higher pressure column 1050 to anintermediate location of the lower pressure column 1110, and the entirenitrogen reflux to the lower pressure column 1110 is condensed overheadvapour from the higher pressure column 1050. Additional refrigerationduty in the ASU is provided by expanding a stream 1082 of nitrogen gasin expander 1084 to produce expanded stream 1086 which is then fed tothe main heat exchanger 1012 where it is combined with waste nitrogenfrom stream 1114 and heated to produce stream 1116.

FIG. 4 depicts the prior art process of FIG. 1 with the addition ofexternal refrigeration from a stream 400 that is heated (withoutchanging phase) and is then vented or returned to an external process.

FIG. 4 a depicts the process of FIG. 4 integrated with the ASU of FIG. 3a in which the stream 400 that is fed to the CPU is the exhaust ofexpander 1088 that operates at a warmer temperature than the main ASUexpander 1084. Stream 400 is heated in the CPU and returned to the ASUas stream 402 where it is mixed with waste nitrogen from stream 1114 inthe main heat exchanger 1012 to be fully reheated to ambient conditionsas stream 1116. Stream 1116 is then used to regenerate the sorbentbed(s) of the front end purification unit 1006.

FIG. 5 depicts the process of FIG. 1 with the addition of externalrefrigeration provided to recirculating carbon dioxide liquid. A part500 of the carbon dioxide liquid removed from the first phase separator112 is pumped in pump 502 to produce pumped stream 504 which leaves theCPU. The stream returns as cooled stream 506, which is reheated in themain exchanger 162 and returned to separator 112.

FIG. 5 a depicts the process of FIG. 5 integrated with the ASU of FIG. 4a. Additional refrigeration is provided at the warm end of the ASU byexpanding a part 1087 of the pressurised nitrogen in a warm expander1088. This additional refrigeration is used to cool stream 504 from theCPU before returning it as cooled stream 506.

COMPARATIVE EXAMPLE 1

Computer simulations using the ASPEN™ Plus software (version 7.2; @Aspen Technology, Inc.) have been carried out to calculate key heat andmass balance and power consumption data for both CPU and ASU depicted inFIG. 1. The simulations were based on an oxyfuel coal fired powerstation with a nominal net electrical output of 500 MW.

As indicated in Table 1, the total ASU and CPU power consumption forthese arrangements is 134400 kW.

TABLE 1 CPU line No. 108 110 122 124 130 140 150 152 160 Temperature °C. 20.0 −31.5 18.5 −53.5 18.5 18.5 20.0 19.9 18.5 Pressure Bar (MPa)30.0 (3) 29.9 (3) 17.6 (1.8) 29.8 (3) 29.7 (3) 7.9 (0.8) 110.0 (11)110.0 (11) 110.0 (11) Molar Flow kmol/s 4.058 4.058 2.015 1.915 1.1800.735 2.751 2.879 0.128 Vapour Fraction 1.0 0.5 1.0 0.6 1.0 1.0 0.0 0.00.0 Mole fraction (CO₂) 0.760 0.760 0.970 0.524 0.253 0.959 0.967 0.9670.970 Mole fraction (O₂) 0.063 0.063 0.009 0.123 0.193 0.011 0.009 0.0090.009 Mole fraction (N₂) 0.153 0.153 0.018 0.304 0.478 0.025 0.020 0.0200.018 Mole fraction (Ar) 0.025 0.025 0.003 0.048 0.076 0.005 0.004 0.0040.003 ASU line No. 200 1008 1044 1116 1132 Temperature ° C. 5.1 10.010.0 5.1 5.1 Pressure Bar 2.1 2.6 4.4 1.1 1.1 (MPa) (0.2) (0.3) (0.4)(0.1) (0.1) Molar Flow kmol/s 0.000 18.391 8.822 14.442 3.949 VapourFraction 1 1 1 1 1 Mole fraction (O₂) 0.016 0.210 0.210 0.007 0.950 Molefraction (N₂) 0.980 0.781 0.781 0.989 0.020 Mole fraction (Ar) 0.0040.009 0.009 0.004 0.030 CPU power 63300 kW ASU Power 71100 kW TotalASU + CPU power 134400 kW 

COMPARATIVE EXAMPLE 2

Similar computer simulations have been carried out to calculate key heatand mass balance and power consumption data for the unintegrated systemdepicted in FIG. 2 a. The simulations were again based on an oxyfuelcoal fired power station with a nominal net output of 500 MW.

As indicated in Table 2, the total ASU and CPU power consumption forthis arrangement is 137150 kW which represents a 2% increase in totalconsumption over the prior art process of FIG. 1. This comparativeexample demonstrates that the reverse Brayton refrigeration cycle isless efficient than the reverse Rankine cycle in which all therefrigeration is provided by evaporating carbon dioxide.

TABLE 2 CPU Line No. 108 110 122 124 130 140 150 152 160 200 202 208Temper- ° C. 20.0 −37.1 11.7 −53.5 11.7 11.7 20.0 18.2 11.7 6.8 −20.511.7 ature Pressure Bar 30.0 (3) 29.9 (3) 18.4 (2) 29.8 (3) 29.7 (3)13.1 (1) 110.0 (11) 110.0 (11) 110.0 (11) 2.6 (0.3) 2.5 (0.3) 1.0 (0.1)(MPa) Molar kmol/s 4.1 4.1 1.8 1.6 1.2 0.4 2.3 2.9 0.6 4.5 4.5 4.5 FlowVapour 1.0 0.4 1.0 0.7 1.0 1.0 0.0 0.0 0.0 1.0 1.0 1.0 Fraction Mol.0.760 0.760 0.966 0.442 0.253 0.959 0.965 0.965 0.966 0.000 0.000 0.000fraction (CO₂) Mole 0.063 0.063 0.010 0.145 0.193 0.011 0.010 0.0100.010 0.012 0.012 0.012 fraction (O₂) Mole 0.153 0.153 0.021 0.357 0.4780.025 0.021 0.021 0.021 0.983 0.983 0.983 fraction (N₂) Mole 0.025 0.0250.004 0.057 0.075 0.005 0.004 0.004 0.004 0.004 0.004 0.004 fraction(Ar) ASU Line No. 200 1008 1044 1116 1132 Temperature ° C. 5.1 10.0 10.05.1 5.1 Pressure Bar 2.1 2.6 4.4 1.1 1.1 (MPa) Molar Flow kmol/s 0.000018.3911 8.8224 14.4419 3.9492 Vapour Fraction 1 1 1 1 1 Mole fraction(O₂) 0.016 0.210 0.210 0.007 0.950 Mole fraction (N₂) 0.980 0.781 0.7810.989 0.020 Mole fraction (Ar) 0.004 0.009 0.009 0.004 0.030 CO2compressor and pump power 58380 kW Nitrogen compressor power 13370 kWNitrogen expander power −5700 kW Total CPU power 66050 kW ASU Power71100 kW Total ASU + CPU power 137150 kW 

EXAMPLE 1

Similar computer simulations have been carried out to calculate key heatand mass balance and power consumption data for the integrated systemdepicted in FIG. 2 b. The simulations were again based on an oxyfuelcoal fired power station with a nominal net output of 500 MW.

As indicated in Table 3, the total ASU and CPU power consumption forthis arrangement is 132540 kW which represents a significant reductionof 1.4% in total consumption over the prior art process of FIG. 1. Thisexample demonstrates that efficiency of the ASU process is increased bywithdrawing pressurised nitrogen from the ASU, and this more thanoffsets the lower efficiency of the refrigeration system in the CPU.

TABLE 3 CPU line No. 108 110 122 124 130 140 150 152 160 200 202 208Temper- ° C. 20.0 −37.1 11.7 −53.5 11.7 11.7 20.0 18.2 11.7 6.8 −20.511.7 ature Pressure Bar 30.0 (3) 29.9 (3) 18.4 (2) 29.8 (3) 29.7 (3)13.1 (1) 110.0 (11) 110.0 (11) 110.0 (11) 2.6 (0.3) 2.5 (0.3) 1.0 (0.1)(MPa) Molar kmol/s 4.058 4.058 1.834 1.598 1.171 0.427 2.261 2.887 0.6264.493 4.493 4.493 Flow Vapour 1.00 0.39 1.00 0.73 1.00 1.00 0.00 0.000.00 1.00 1.00 1.00 Fraction Mole 0.760 0.760 0.966 0.442 0.253 0.9590.965 0.965 0.966 0.000 0.000 0.000 fraction (CO₂) Mole 0.063 0.0630.010 0.145 0.193 0.011 0.010 0.010 0.010 0.012 0.012 0.012 fraction(O₂) Mole 0.153 0.153 0.021 0.357 0.478 0.025 0.021 0.021 0.021 0.9830.983 0.983 fraction (N₂) Mole 0.025 0.025 0.004 0.057 0.075 0.005 0.0040.004 0.004 0.004 0.004 0.004 fraction (Ar) ASU line No. 200 1008 10441116 1132 Temperature ° C. 6.8 10.0 10.0 6.8 6.8 Pressure Bar 2.7 (0.3)3.0 (0.3) 4.4 (0.4) 1.1 (0.1) 1.1 (0.1) (MPa) Molar Flow kmol/s 4.49319.329 8.642 10.882 3.953 Vapour Fraction 1 1 1 1 1 Mole fraction (O₂)0.012 0.210 0.210 0.022 0.950 Mole fraction (N₂) 0.983 0.781 0.781 0.9730.023 Mole fraction (Ar) 0.004 0.009 0.009 0.005 0.027 CO2 compressorand pump power 58380 kW Nitrogen expander power −5700 kW Total CPU power52680 ASU Power 79860 kW Total ASU + CPU power 132540 kW

It will be appreciated that the invention is not restricted to thedetails described above with reference to the preferred embodiments butthat numerous modifications and variations can be made without departingform the spirit or scope of the invention as defined in the followingclaims.

1. A method of purifying crude carbon dioxide gas containing at leastone non-condensible gas contaminant, under elevated pressure in a carbondioxide purification unit (“CPU”), said method comprising: feeding crudecarbon dioxide gas to the CPU; cooling and condensing carbon dioxide gasfrom the crude carbon dioxide gas; and separating condensed carbondioxide gas from said non-condensible gas contaminant(s) to produce atleast one carbon dioxide liquid and a tail gas comprising saidnon-condensible gas contaminant(s), wherein the method requiresrefrigeration duty for cooling and condensing carbon dioxide gas, saidrefrigeration duty being provided in at least a first part by indirectheat exchange against at least one liquid first refrigerant therebyevaporating said first refrigerant(s), and a second part by indirectheat exchange against sensible heat energy alone of at least one secondrefrigerant.
 2. The method according to claim 1, wherein at least aportion of said first part of the refrigeration duty is provided by atleast one of said carbon dioxide liquid(s) as said liquid firstrefrigerant(s).
 3. The method according to claim 1, wherein a furtherpart of said refrigeration duty is provided by indirect heat exchangeagainst said tail gas.
 4. The method according to claim 3, wherein saidfurther part is no more than 10% of said refrigeration duty.
 5. Themethod according to claim 3, wherein said first and second parts providethe remainder of said refrigeration duty.
 6. The method according toclaim 1, wherein the second refrigerant has a temperature that issufficiently low to cool said crude carbon dioxide gas to below the CO₂dew point temperature, and preferably close to the CO₂ triple pointtemperature.
 7. The method according to claim 1, wherein the secondrefrigerant has a temperature that is sufficiently low to cool saidcrude carbon dioxide gas to a temperature from about −56° C. to about10° C.
 8. The method according to claim 1, wherein said refrigerationduty has a colder part that both cools and condenses said crude carbondioxide gas and a warmer part that cools the crude carbon dioxide gaswith no condensation, said second refrigerant providing at least aportion of said colder part.
 9. The method according to claim 1, whereinsaid second refrigerant is used in a quantity that is no more than thatsufficient to optimize power consumption.
 10. The method according toclaim 1, wherein said second refrigerant provides no more than 30%,preferably no more than 20%, of the refrigeration duty.
 11. The methodaccording to claim 1, wherein said second refrigerant is independent ofsaid crude carbon dioxide gas.
 12. The method according to claim 1,wherein the second refrigerant is derived from said crude carbon dioxidegas and exported from the CPU for cooling.
 13. The method according toclaim 1, wherein said second refrigerant is imported into the CPU. 14.The method according to claim 1, wherein said second refrigerant is agas.
 15. The method according to claim 1, wherein said secondrefrigerant is nitrogen gas.
 16. The method according to claim 1,wherein the CPU is integrated with a cryogenic air separation unit(“ASU”) and the second refrigerant is nitrogen gas imported from theASU.
 17. The method according to claim 1, comprising: importing apressurized gas into the CPU; and expanding said pressurized gas in saidCPU after optionally cooling said gas, to produce said secondrefrigerant.
 18. The method according to claim 1, comprising: separatingcompressed air feed by cryogenic distillation in an ASU to producenitrogen gas under pressure and gaseous oxygen; importing at least aportion of said nitrogen gas into the CPU; cooling said importednitrogen gas by indirect heat exchange to form cooled nitrogen gas;expanding said cooled nitrogen gas in the CPU to produce expandednitrogen gas; and using said expanded nitrogen gas as said secondrefrigerant to produce warmed nitrogen gas.
 19. The method according toclaim 18, wherein said imported nitrogen gas is cooled by indirect heatexchange against said liquid first refrigerant(s) and/or said secondrefrigerant and/or said tail gas, to form said cooled nitrogen gas. 20.The method according to claim 18, wherein the imported nitrogen gas iscooled to a temperature in the range from about −40° C. to about 10° C.21. The method according to claim 18, wherein the cooled nitrogen gas isexpanded to a pressure from about 1 bar to about 4 bar.
 22. The methodaccording to claim 18, wherein a portion of the carbon dioxide liquid(s)is removed from the CPU as a liquid product and optionally pumped andwarmed to ambient temperature.
 23. The method according to claim 18,wherein said cooled nitrogen gas is expanded in a first expander, warmedand then further expanded in a second expander.
 24. The method accordingto claim 1, comprising importing said second refrigerant into the CPUand using said second refrigerant directly to provide said second partof the refrigeration duty.
 25. The method according to claim 1,comprising: separating compressed air feed by cryogenic distillation inan ASU to produce nitrogen gas under pressure and gaseous oxygen;expanding at least a portion of said nitrogen gas in the ASU to produceexpanded nitrogen gas; importing said expanded nitrogen gas into theCPU; and using said expanded nitrogen gas as said second refrigerant toproduce warmed nitrogen gas.
 26. The method according to claim 25,comprising: removing water and carbon dioxide from compressed air in apurification unit having at least one sorbent bed to produce saidcompressed air feed for the ASU; and regenerating said sorbent bed(s)using at least a portion of said warmed nitrogen gas.
 27. The methodaccording to claim 1 wherein the second refrigerant is a liquid.
 28. Themethod according to claim 1, wherein the second refrigerant is carbondioxide liquid.
 29. The method according to claim 1, wherein the secondrefrigerant is at least a portion of said carbon dioxide liquid(s). 30.The method according to claim 1, wherein said CPU comprises two phaseseparators, said method comprising: cooling and condensing carbondioxide gas in the crude carbon dioxide gas; separating the condensedcarbon dioxide gas from said non-condensible gas contaminant(s) in afirst phase separator to produce a first carbon dioxide liquid and afirst overhead vapour comprising said non-condensible gascontaminant(s); dividing said first carbon dioxide liquid into threeportions; pumping a first portion of said first carbon dioxide liquid toproduce a pumped first portion; reducing the pressure of a secondportion of said first carbon dioxide liquid to produce a reducedpressure second portion; pumping a third portion of said first carbondioxide liquid to produce a pumped third portion, and cooling the pumpedthird portion to produce a cooled third portion; cooling and condensingcarbon dioxide gas in said first overhead vapour; separating thecondensed carbon dioxide gas from said non-condensible gascontaminant(s) in a second phase separator to produce a second carbondioxide liquid and said tail gas comprising said non-condensible gascontaminant(s); warming said tail gas by indirect heat exchange toproduce warmed tail gas; and reducing the pressure of said second carbondioxide liquid after optionally warming said liquid, to produce reducedpressure second carbon dioxide liquid, wherein the liquid firstrefrigerant(s) providing the first part of the refrigeration duty arethe pumped first portion of first carbon dioxide liquid, the reducedpressure second portion of first carbon dioxide liquid and the reducedpressure second carbon dioxide liquid, and wherein the secondrefrigerant is the cooled third portion of the first carbon dioxideliquid.
 31. The method according to claim 30, wherein the pumped thirdportion of the first carbon dioxide liquid is cooled by indirect heatexchange against expanded nitrogen gas in an integrated ASU.
 32. Themethod according to claim 1, wherein the CPU is integrated with anoxyfuel combustion unit producing a net flue gas that is compressed anddried to produce said crude carbon dioxide gas.
 33. The method accordingto claim 30, wherein oxygen for the oxyfuel combustion unit is suppliedfrom an ASU integrated with the CPU.
 34. The method according to claim1, wherein at least a portion of the carbon dioxide liquid(s) is removedfrom the CPU and stored as a liquid.
 35. The method according to claim1, wherein at least a portion of the carbon dioxide liquid(s) is removedfrom the CPU, pumped and optionally heated to ambient temperature.
 36. Amethod for generating power, said method comprising: combusting a fuelselected from the group consisting of carbonaceous fuels,hydrocarbonaceous fuels and biomass, in an oxygen-rich atmosphere withinan oxyfuel combustion unit to produce heat and flue gas; recovering atleast a portion of said heat to generate said power; dividing said fluegas after optionally desulfurizing said flue gas, into recycle flue gasand net flue gas; recycling said recycle flue gas to the oxyfuelcombustion unit; compressing and drying said net flue gas to produce dryflue gas under elevated pressure containing at least one non-condensiblecontaminant; feeding said dry flue gas to a CPU wherein carbon dioxidegas from said dry flue gas is cooled and condensed, and separated fromsaid non-condensible gas contaminant(s) to produce at least one carbondioxide liquid and a tail gas comprising said non-condensible gascontaminants; separating compressed air by cryogenic distillation in anASU to produce nitrogen gas under elevated pressure and gaseous oxygen;feeding at least a portion of said gaseous oxygen to said oxyfuelcombustion unit; feeding at least a portion of said nitrogen gas to theCPU wherein the nitrogen gas is cooled by indirect heat exchange toproduce cooled nitrogen gas which is then expanded to produce expandednitrogen gas; wherein the method in the CPU requires refrigeration duty,said refrigeration duty being provided in at least a first part byindirect heat exchange against at least a portion of said carbon dioxideliquid(s) thereby evaporating said liquid(s), and a second part byindirect heat exchange against sensible heat energy alone of saidexpanded nitrogen gas.
 37. The method according to claim 36, wherein theimported nitrogen gas is cooled to a temperature in the range from about−40° C. to about 10° C.
 38. The method according to claim 36, whereinthe cooled nitrogen gas is expanded to a pressure in the range fromabout 1 bar to about 4 bar.
 39. The method according to claim 36,wherein a portion of the carbon dioxide liquid(s) is removed from theCPU as a liquid product and optionally pumped and warmed to ambienttemperature.
 40. The method according to claim 36, wherein said coolednitrogen gas is expanded in a first expander, warmed and then furtherexpanded in a second expander to produce said expanded nitrogen gas. 41.A method for generating power, said method comprising: combusting a fuelselected from the group consisting of carbonaceous fuels,hydrocarbonaceous fuels and biomass, in an oxygen-rich atmosphere withinan oxyfuel combustion unit to produce heat and flue gas; recovering atleast a portion of said heat to generate said power; dividing said fluegas after optionally desulfurizing said flue gas, into recycle flue gasand net flue gas; recycling said recycle flue gas to the oxyfuelcombustion unit; compressing and drying said net flue gas to produce dryflue gas under elevated pressure containing at least one non-condensiblecontaminant; feeding said dry flue gas to a CPU wherein carbon dioxidegas from said dry flue gas is cooled and condensed, and separated fromsaid non-condensible gas contaminant(s) to produce at least one carbondioxide liquid and a tail gas comprising said non-condensible gascontaminants; separating compressed air by cryogenic distillation in anASU to produce nitrogen gas under elevated pressure and gaseous oxygen;feeding at least a portion of said gaseous oxygen to said oxyfuelcombustion unit; expanding in the ASU at least a portion of saidnitrogen gas to produce expanded nitrogen gas; and feeding said expandednitrogen gas to said CPU; wherein the method in the CPU requiresrefrigeration duty, said refrigeration duty being provided in at least afirst part by indirect heat exchange against at least a portion of saidcarbon dioxide liquid(s) thereby evaporating said liquid(s), and asecond part by indirect heat exchange against sensible heat energy aloneof said expanded nitrogen gas.
 42. A method for generating power, saidmethod comprising: combusting a fuel selected from the group consistingof carbonaceous fuels, hydrocarbonaceous fuels and biomass, in anoxygen-rich atmosphere within an oxyfuel combustion unit to produce heatand flue gas; recovering at least a portion of said heat to generatesaid power; dividing said flue gas after optionally desulfurizing saidflue gas, into recycle flue gas and net flue gas; recycling said recycleflue gas to the oxyfuel combustion unit; compressing and drying said netflue gas to produce dry flue gas under elevated pressure containing atleast one non-condensible contaminant; feeding said dry flue gas to aCPU for purification, said purification comprising: cooling andcondensing carbon dioxide gas in the dry flue gas; separating thecondensed carbon dioxide gas from said non-condensible gascontaminant(s) in a first phase separator to produce a first carbondioxide liquid and a first overhead vapour comprising saidnon-condensible gas contaminant(s); dividing said first carbon dioxideliquid into three portions; pumping a first portion of said first carbondioxide liquid to produce a pumped first portion; reducing the pressureof a second portion of said first carbon dioxide liquid to produce areduced pressure second portion; pumping a third portion of said firstcarbon dioxide liquid to produce a pumped third portion, and cooling thepumped third portion to produce a cooled third portion; cooling andcondensing carbon dioxide gas in said first overhead vapour; separatingthe condensed carbon dioxide gas from said non-condensible gascontaminant(s) in a second phase separator to produce a second carbondioxide liquid and said tail gas comprising said non-condensible gascontaminant(s); warming said tail gas by indirect heat exchange toproduce warmed tail gas; and reducing the pressure of said second carbondioxide liquid after optionally warming said liquid, to produce reducedpressure second carbon dioxide liquid, separating compressed air bycryogenic distillation in an ASU to produce nitrogen gas under elevatedpressure and gaseous oxygen; feeding at least a portion of said gaseousoxygen to said oxyfuel combustion unit; and expanding at least a portionof said nitrogen gas to produce expanded nitrogen gas; wherein themethod in the CPU requires refrigeration duty, said refrigeration dutybeing provided in at least a first part by indirect heat exchange withthe pumped first portion of first carbon dioxide liquid, the reducedpressure second portion of first carbon dioxide liquid and the reducedpressure second carbon dioxide liquid, and a second part by indirectheat exchange with sensible heat energy alone of the cooled thirdportion of the first carbon dioxide liquid.
 43. The method according toclaim 42, wherein the pumped third portion of the first carbon dioxideliquid is cooled by indirect heat exchange against said expandednitrogen gas.